What is it?

Hi I have a shrub growing profusely through the hedge beside my house with very hard green fruits about marble size and I was wondering if anyone can tell me what it is? I have searched on Google without success. Thanks in advance.

I think it could be a bullace - another sort of wild plum, like a damson but with rounder, sweeter fruit. They used to grow wild near my granny's house in rural Essex, and she made them into jam and pies. There's pics on Google...